Nand flash memory having multiple cell substrates

ABSTRACT

A NAND flash memory bank having a plurality of bitlines of a memory array connected to a page buffer, where NAND cell strings connected to the same bitline are formed in at least two well sectors. At least one well sector can be selectively coupled to an erase voltage during an erase operation, such that unselected well sectors are inhibited from receiving the erase voltage. When the area of the well sectors decrease, a corresponding decrease in the capacitance of each well sector results. Accordingly, higher speed erasing of the NAND flash memory cells relative to a single well memory bank is obtained when the charge pump circuit drive capacity remains unchanged. Alternately, a constant erase speed corresponding to a single well memory bank is obtained by matching a well segment having a specific area to a charge pump with reduced drive capacity. A reduced drive capacity charge pump will occupy less semiconductor chip area, thereby reducing cost.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/143,285, filed on Jun. 20, 2008, which claims the benefit of priorityof U.S. Provisional Patent Application No. 61/019,415 filed on Jan. 7,2008, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to NAND flash memory. Moreparticularly, the present invention relates to erasing NAND flash memorycells.

BACKGROUND

Flash memory is a commonly used type of non-volatile memory inwidespread use as storage for consumer electronics and mass storageapplications. Flash memory is pervasive in popular consumer productssuch as digital audio/video players, cell phones and digital cameras,for storing application data and/or media data. Flash memory can furtherbe used as a dedicated storage device, such as a portable flash drivepluggable into a universal serial port (USB) of a personal computer, anda magnetic hard disk drive (HDD) replacement for example. It is wellknown that flash memory is non-volatile, meaning that it retains storeddata in the absence of power, which provides a power savings advantagefor the above mentioned consumer products. Flash memory is suited forsuch applications due to its relatively high density for a given area ofits memory array.

FIG. 1A is a general block diagram of typical flash memory device. Flashmemory 2 includes well known input and output buffer circuits, such asinput/output (I/O) buffer block 3 a and control buffer block 3 b forreceiving external control and data input signals and providing dataoutput signals. The control buffer block 3 b receiving the controlsignals, such as CE# and WE#, may include other basic logic circuits,for implementing rudimentary functions that may be related to control ofthe data input and buffers for example. Flash memory 2 includes controlcircuit 3 c, for controlling various high level functions of the flashcircuits such as read, program and erase operations for example, anaddress register 4 for storing address information, a data register 5for storing program data information, a command register 6 for storingcommand data information, high voltage circuits for generating therequired program and erase voltages, and core memory circuits foraccessing the memory array 7. Memory array 7 includes flash memorycells, arranged as NAND cell strings for example. The NAND cell stringsof a column are coupled to a bitline, which is connected to a pagebuffer/sense amplifier circuit 8. Sense amplifier circuit 8 senses readdata from a selected page of memory cells and provides program data to aselected page of memory cells. One page of memory cells refers to allthe memory cells connected to the same wordline. Driving the wordlinesis row drivers/decoders, shown as a row address decoder 9 a and rowaddress buffer 9 b. There can be one or more stages of decoding, and rowaddress buffer 9 b can include block decoding logic.

The control circuit 3 c includes a command decoder and logic forexecuting internal flash operations, such as read, program and erasefunctions. Those skilled in the art will understand that theseoperations are executed in response to the command data stored in thecommand register 6, sometimes in combination with the address data andprogram data stored in the respective address register 4 and dataregister 5, depending on the operation to be executed. The command data,address data and program data are issued by a memory controller andlatched into the corresponding registers by flash memory 2. Thefunctions of the shown circuit blocks of flash memory 2 are well knownin the art. Persons skilled in the art will understand that flash memory2 shown in FIG. 1A represents one possible flash memory configurationamongst many possible configurations. In FIG. 1A, memory array 7, senseamplifier circuit 8, data register 5, row address decoder 9 a and rowaddress buffer 9 b are part of one memory bank.

FIG. 1B is a floor plan layout a prior art flash memory device to showthe area occupied by various circuit blocks. Typically, all the circuitblocks shown in FIG. 1A are formed in the floor plan layout of FIG. 1B.In FIG. 1B, flash memory chip 10 is a semiconductor material rectangularin shape, upon which are formed transistor circuits and structures.Occupying a large proportion of the area are two memory arrays or memorytiles, 12 and 14, which generally correspond to memory array 7 of FIG.1A. While the present example flash memory 10 includes two memoryarrays, alternative designs can include a single memory array or morethan two memory arrays. Located between memory arrays 12 and 14 are rowdecoders 16 that drive wordlines to the required voltage level for read,program and erase operations. Row decoders 16 generally correspond torow address decoder 9 a and row address buffer 9 b of FIG. 1A. In theexample of FIG. 1B, wordlines (not shown) extend in a horizontaldirection. Located below each of memory arrays 12 and 14 are pagebuffers 18 and 20, each being electrically connected to bitlines (notshown) for providing program data and for sensing read data. Pagebuffers 18 and 20 generally correspond to data register 5 and senseamplifier 8 of FIG. 1A. The combination of memory array 12, row decoders16 and page buffer 18 is referred to as a memory bank or plane.Similarly, the combination of memory array 14, row decoders 16 and pagerbuffer 20 is referred to as another memory bank or plane. The pagebuffers 18 and 20 receive and provide data via data lines (not shown),which are coupled to the input and output (I/O) circuits in logic block22. Logic block 22 further includes other circuits such as a commanddecoder and registers. Another large area is dedicated for a charge pump24, which is responsible for generating high voltages required forprogramming and erasing data stored in the flash memory cells of thefirst memory array 12 and the second memory array 14. Charge pump 24generally corresponds to the high voltage generator of FIG. 1A. Theelements of flash memory chip 10 have been generically described, butpersons skilled in the art will understand that each of the outlinedblocks of FIG. 1B will include all the circuits necessary to achieveproper operation of flash memory chip 10.

In the presently shown example of FIG. 1B, the flash memory chip 10 isdesigned to have NAND flash memory cells arranged in NAND cell stringswithin memory arrays 12 and 14. The NAND cell strings are organized intomemory blocks, such as Block[1] to Block[n], where n can be any non-zerointeger value. The selection of the number of blocks in each array is adesign parameter of flash memory chip 10.

FIG. 2 depicts an example memory array of flash memory chip 10 of FIG.1B. The example illustrated in FIG. 2 has two memory blocks in onememory array. In FIG. 2, one NAND cell string is outlined with a dashedbox 30, which includes a string select device 32, flash memory cells 34,and a sourceline select device 36 connected in series between bitlineBL1 and common source line CSL. There can be “i” flash memory cells 34per NAND cell string, where i is a non-zero integer value. Accordingly,wordlines WL1 to WLi are electrically coupled to corresponding gates ofthe flash memory cells 34. A string select line (SSL) and a sourceselect line (GSL) are electrically coupled to select devices 32 and 36respectively. In the present example, all the transistors of the NANDcell string 30 are n-channel devices.

A memory block 38, being the same as memory Block[1] of FIG. 1B forexample, will include all the NAND cell strings having select devicesand flash memory cells connected to the same wordlines, string selectline and source select line. The width of memory block 38 is set by thenumber of bitlines, which in the case of FIG. 2 is “j” bitlines where jis a non-zero integer value. Memory block 40 includes further NAND cellstrings connected to bitlines BL1 to BLj. A bitline and the NAND cellstrings electrically connected to it is referred to as a column.

All the circuits of flash memory chip 10 of FIG. 1B, including the NANDcell strings shown in FIG. 2 are formed by using well-knownsemiconductor manufacturing processes. In such processes, transistors ofthe same type are grouped together and formed in their own well. Forexample, n-type transistors are formed in a p-type well and p-typetransistors are formed in an n-type well. In some cases, only a singlewell is used, where its type depends on the type of the substrate. Inmost NAND flash memory devices, all the NAND cell strings in a memoryarray are formed in one well, which results in disadvantages that aredescribed later on.

FIG. 3 is a cross-sectional diagram of memory array 14 taken along lineA-A′ of FIG. 1B, and angled to show specific features on its surface.The cross-sectional structure of the semiconductor substrate where pagebuffer 20 and logic block 22 are formed is not shown. In FIG. 3, thesubstrate 50 is a p-type substrate having an n-well 52 and a p-well 54.P-well 54 is formed within n-well 52 such that p-well 54 is spaced fromsubstrate 50. All the NAND cell strings 30 of FIG. 2, and morespecifically the transistor devices of NAND cell strings 30, are formedwithin p-well 54. The well structure shown in FIG. 3 is commonly knownas a triple-well structure, or a triple pocket structure. On the surfaceof p-well 54 are the NAND cell strings 30, simply represented astrapezoid boxes, where each NAND cell string of a column is connected inparallel to a bitline, such as bitline BLk where “k” is a variablerepresenting a logical bitline position less than BLj. With reference toFIG. 2, the bitline is connected to the string select device 32 of eachNAND cell string 30. Accordingly, the NAND cell strings that sharecommon select lines and wordlines are part of one memory block. FIG. 3illustrates four memory blocks 56, 58, 60 and 62 to simply the drawing,however those skilled in the art will understand that there can be anynumber of memory blocks in memory arrays 12 and 14. Both the n-well 52and the p-well 54 receives an erase voltage Verase during eraseoperations, and are both biased to 0V or VSS during all other operationssuch as program and read for example. Verase can be coupled to n-well 52and p-well 54 at multiple different locations.

FIG. 4 is a cross section diagram of a NAND cell string 30 of FIG. 3,having the equivalent circuit diagram shown in FIG. 2. Each flash memorycell includes a polysilicon wordline 70 and a polysilicon floating gate72, where the floating gate 72 is formed over a thin gate oxide 74. Oneither side of thin gate oxide 74 and formed within p-type well 54 aren-type diffusion regions 76. The sourceline select device 36 includes apolysilicon gate 78 formed over a thick gate oxide 80, and an n-typediffusion region 82 acting as the common source line CSL. Diffusionregion 82 is shared with all the NAND cell strings in the memory block,as illustrated in FIG. 2. The string select device 32 includes apolysilicon gate 84 formed over a thick gate oxide 86, and an n-typediffusion region 88 that is electrically connected to a bitline 90.

As is well known in the art, NAND flash memory devices are blockerasable, meaning that individual memory blocks can be selectivelyerased through Fowler-Nordheim (F-N) tunneling, based on a block addressor other selection signal. In order to erase a memory block such asmemory block 38 of FIG. 2, the wordlines of the selected memory blockare biased to 0V, SSL and GSL are floated, and both the n-well 52 andthe p-well 54 are biased to Verase. Verase is a high voltage generatedby the charge pump 24 of FIG. 1B, and in example flash memory devices isabout 20V. Because SSL and GSL are floated during the erase operation,both SSL and GSL are self-boosted when Verase is applied to n-well 52and p-well 54 due to the capacitive coupling between the wells and SSLand GSL. Depending on the capacitive coupling ratio, GSL and SSL can beboosted to approximately 80% to 90% of Verase. CSL and all bitlines arefloated during the erase operation, and eventually self-boost to aboutVerase-0.6V. Those skilled in the art will understand that the forwardbias p-n junction voltage drop across p-well 54 to the n-type diffusionregions 82 and 88. Under these erase bias conditions, trapped electrons(charge) in the floating gate of the flash memory cells are emitteduniformly to the substrate. The threshold voltage (Vth) of the erasedflash memory cell becomes negative, meaning that the erased cell willturn on with a gate bias of 0V.

Since the unselected memory blocks reside in the same p-well 54 as theselected memory block, these unselected memory blocks must be inhibitedfrom being erased. A self-boosting erase inhibit scheme described inU.S. Pat. No. 5,473,563 is widely used in NAND flash memory devices toprevent erasure of unselected memory blocks. To prevent erasure of flashmemory cells in unselected memory blocks using the self-boosting eraseinhibit scheme, all wordlines in unselected memory blocks are floated.Therefore floated wordlines in the unselected memory blocks are boostedto about 90% of Verase when the p-well 54 rises to Verase, by capacitivecoupling between the p-well 54 and the wordlines. It should beunderstood that the final boosted voltage level on the floatingwordlines is determined by the coupling ratio between the substrate andwordlines. The boosted voltage of the wordlines in the unselected memoryblocks is effective for reducing the electric field between the p-well54 and the wordlines, thereby minimizing unintended erasure of datastored therein.

Once the erase operation ends, Verase is set to VSS for a block eraseverify operation for determining if all the flash memory cells of theselected memory block have been successfully erased. If not, then asubsequent erase operation is executed upon the selected memory block.Verase is also set to VSS during read and program operations, oralternately, a different circuit couples VSS to n-well 52 and p-well 54.For example, n-channel transistor devices can be used to couple n-well52 and p-well 54 in response to a control signal that is activatedduring read or program operations. Logic for executing such an operationwould be well known to those skilled in the art. A problem with theprior art NAND flash memory is the amount of time required to driven-well 52 and p-well 54 from VSS to Verase, which directly affects thetotal erase time. It is apparent from FIG. 1B that the area of onememory bank is large relative to the total area of flash memory chip 10,and thus the capacitance can be in the range of several nF for example.As a result, the rise time of Verase can be between 200 μs to 300 μs,for example.

FIG. 5 is a graph plotting the relationship between the substratevoltage Vsub and time. If an erase operation begins at time=0 and Veraseis at VSS, then there is a delay of t_delay before the substrate voltagereaches Verase. As previously mentioned, this delay can range between200 μs to 300 μs for some example flash memory devices. A solution toimprove erase performance is to increase the size of the charge pumpcircuit that generates Verase. This typically involves a combination ofadding capacitor elements or increasing the size of capacitor elementsof the charge pump to increase the rate at which the substrate reachesVerase. A larger charge pump would thus reduce t_delay and improve eraseperformance. Persons skilled in the art understand that capacitorelements used in such charge pumps occupy significant semiconductorarea. FIG. 1B clearly shows that charge pump 24 occupies a significantarea of flash memory chip 10, especially in comparison with the logicblock 22. An example charge pump circuit is shown in U.S. Pat. No.5,642,309. In view of the tightly packed layout of the example flashmemory chip 10 of FIG. 1B, there is insufficient area for increasing thesize of charge pump 24. Accordingly, improved erase performance in flashmemory chip 10 may not be attained. In some flash memory chip designs,the primary constraint may be to minimize chip size, which directlyimpacts the cost of the chip. While a minimally sized charge pump willreduce chip area consumption, the drawback is degraded eraseperformance. Hence there is a trade-off between erase performance andchip area in prior art flash memory chips.

Another problem with the prior art NAND flash memory is the powerconsumption due to the charging and discharging of the n-well 52 andp-well 54. As previously mentioned, because each of the memory arraywells occupy a large proportion of the area of flash memory chip 10 ofFIG. 1B, their capacitance can be in the range of several nF forexample. This is problematic because after each erase cycle, an eraseverify operation is executed to check that the erased memory cells havethe erased threshold voltage. An erase verify operation is similar to anormal NAND flash read operation, and therefore the n-well 52 and p-well54 are biased to VSS. If the verify operation fails, then the erasecycle is repeated and the wells are charged back to Verase. This processmay repeat several times, thus consuming power.

A further problem with the prior art NAND flash memory is the exposureof unselected memory blocks to the Verase well voltage when a selectedmemory block is to be erased. Although the previously describedself-boosting erase inhibit scheme can be used to minimize erasedisturbance in the cells of the unselected memory blocks, there is stilla voltage difference between Verase of the well and the wordlines thatare at about 80% to 90% of Verase in the unselected blocks. While theresulting erase disturb may be small for one erase cycle, the cumulativeeffect will be significant. For example, if it is assumed that thememory array has 2048 memory blocks and the erase time for one memoryblock is about 2 ms, then erasing all the memory blocks just once willexpose each memory block to 2047×2 ms of erase stress. The cumulativeerase disturb stress is more significant in multi-level NAND flashcells.

SUMMARY

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous NAND Flash memories.

According to an embodiment of the present invention achieves a NANDflash memory chip having high speed erase performance while minimizingcharge pump circuit area, power consumption and erase stress forunselected memory blocks.

For example, in accordance with one embodiment, there is provided with aNAND flash memory bank having a plurality of bitlines of a memory arrayconnected to a page buffer, where NAND cell strings connected to thesame bitline are formed in at least two well sectors. At least one wellsector can be selectively coupled to an erase voltage during an eraseoperation, such that unselected well sectors are inhibited fromreceiving the erase voltage. When the area of the well sectors decrease,a corresponding decrease in the capacitance of each well sector results.Accordingly, higher speed erasing of the NAND flash memory cellsrelative to a single well memory bank is obtained when the charge pumpcircuit drive capacity remains unchanged. Alternately, a constant erasespeed corresponding to the single well memory bank is obtained bymatching a well segment having a specific area to a charge pump withreduced drive capacity. A reduced drive capacity charge pump will occupyless semiconductor chip area. Furthermore, because the well sectorcapacitance is reduced, the amount of power consumed for charging anddischarging the well sector during erase operations is also reduced.

In a first aspect, the present invention provides NAND Flash memory. TheNAND flash memory includes a first well sector, a second well sector, abitline and a page buffer. The first well sector has a first NAND cellstring for selectively receiving an erase voltage during an eraseoperation. The second well sector has a second NAND cell string forselectively receiving the erase voltage during the erase operation. Thebitline is electrically connected to the first NAND cell string and thesecond NAND cell string. The page buffer is electrically connected tothe bitline. The first well sector can include a third NAND cell stringelectrically connected to a second bitline, and the second well sectorcan include a fourth NAND cell string electrically connected to thesecond bitline, where the second bitline is electrically connected tothe page buffer. The first NAND cell string and the third NAND cellstring are part of one memory block, and the second NAND cell string andthe fourth NAND cell string are part of another memory block.

Alternately, the first well sector can include a third NAND cell stringelectrically connected to the bitline, and the second well sector caninclude a fourth NAND cell string electrically connected to the bitline.The first NAND cell string is part of a first memory block, the thirdNAND cell string is part of a second memory block, the second NAND cellstring is part of a third memory block, and the fourth NAND cell stringis part of a fourth memory block. The NAND flash memory can furtherinclude a block decoder for selecting one of the first memory block, thesecond memory block, the third memory block and the fourth memory blockfor erasure, in response to a block address. A charge pump and aselector can be provided, where the charge pump provides an erasevoltage and the selector couples the erase voltage to one of the firstwell sector and the second well sector in response to the block address.

The bitline described in the first aspect can include a first bitlinesegment electrically connected to the first NAND cell string and asecond bitline segment electrically connected to the second NAND cellstring through an isolation device, where the isolation device islocated between the first well sector and the second well sector. Theisolation device can have its gate terminal biased to a voltage greaterthan a supply voltage VDD during a program operation, a read operationand the erase operation. Alternately, the isolation device can berendered electrically non-conductive in the erase operation forisolating the first bitline segment from the second bitline segment whenone of the first NAND cell string and the second NAND cell string isselected for erasure. The isolation device can be turned off in responseto a control signal or in response to a well sector selection signal.The NAND flash memory can further include a bitline segment decoder forenabling the isolation device in response to a well sector selectionsignal during a read operation, the bitline segment decoder disablingthe isolation device in response to an erase control signal during anerase operation. The bitline segment decoder can include an isolationdevice driver for receiving the erase control signal and the well sectorselection signal, the isolation device driver providing an isolationdrive signal for controlling the isolation device when the well sectorselection signal is at an active logic level. The isolation devicedriver can include an override circuit for driving the well sectorselection signal to the active logic level in response to another wellsector selection signal at the active logic level.

In a second aspect, the present invention provides a NAND Flash memory.The NAND Flash memory includes at least two well sectors each includingat least one memory block of NAND cell strings, and isolation devices.The at least one memory block in each of the at least two well sectorsis electrically connected to corresponding bitline segments, and theisolation devices are coupled between the bitline segments correspondingto the at least two well sectors. Each of the at least two well sectorscan include two memory blocks. The isolation devices can have gateterminals biased to a predetermined voltage which can be greater than asupply voltage VDD. Alternately, the isolation devices are turned offduring an erase operation, or are selectively turned off during a readoperation. During a read operation the isolation devices between aselected well sector including a selected memory block and a page bufferare turned on. The NAND flash memory further includes a selector forselectively passing an erase voltage to one of the at least two wellsectors. The selector couples the erase voltage to one of the at leasttwo well sectors in response to a portion of a block address, the blockaddress being decoded to select one memory block for erasure. The NANDflash memory can further include a page buffer electrically connected tothe bitline segments corresponding to one of the at least two wellsectors.

In a third aspect, the present invention provides a method for erasing aselected memory block in a NAND Flash device. The method includesselecting a memory block in a first well sector, the first well sectorincluding at least two memory blocks; biasing the memory block formed inthe first well sector for erasure; biasing an unselected memory blockformed in the first well sector for inhibiting erasure; applying anerase voltage to the first well sector; and, inhibiting application ofthe erase voltage to a second well sector including at least another twomemory blocks. The method can further include decoupling bitlinesegments corresponding to the first well sector and the second wellsector from each other before applying the erase voltage to the firstwell sector. Alternately, the method can further include decouplingbitline segments corresponding to the first well sector and the secondwell sector from each other with an isolation device when a bitlinevoltage of the first well sector is at least a predetermined biasvoltage applied to a gate terminal of the isolation device.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1A is a block diagram of a flash memory device;

FIG. 1B is a floor plan layout a prior art flash memory device;

FIG. 2 is a circuit schematic showing circuit details of two memoryblocks in one memory array of the flash memory chip of FIG. 1B;

FIG. 3 is a cross-sectional diagram of one memory array of the flashmemory chip of FIG. 1B;

FIG. 4 is a cross section diagram of a NAND cell string of FIG. 3;

FIG. 5 is a graph plotting the relationship between the substratevoltage Vsub and time;

FIG. 6 is a block diagram of a NAND flash memory bank according to anembodiment of the present invention;

FIG. 7A is a block diagram of a row decoder used in the NAND flashmemory bank of FIG. 6;

FIG. 7B is circuit schematic of a memory block drive circuit shown inFIG. 7A;

FIG. 8A is a block diagram of a NAND flash memory bank having one memoryblock per well sector, according to an example of the NAND flash memorybank of FIG. 6;

FIG. 8B is a cross-sectional diagram of one memory array of the NANDflash memory bank of FIG. 8A;

FIG. 9A is a block diagram of a NAND flash memory bank having multiplememory blocks per well sector, according to another example of the NANDflash memory bank of FIG. 6;

FIG. 9B is a cross-sectional diagram of one memory array of the NANDflash memory bank of FIG. 9A;

FIG. 10 is the cross-sectional diagram of FIG. 9B including isolationdevices formed in-line with the bitlines;

FIG. 11 is a circuit schematic of a memory bank having dynamicallycontrolled isolation devices, according to one example;

FIG. 12 is a circuit schematic of a memory bank having dynamicallycontrolled isolation devices, according to another example;

FIG. 13 is a circuit schematic of a memory bank having dynamicallycontrolled isolation devices, according to yet another example; and,

FIG. 14 is a flow chart showing a method of erasing a memory block,according an embodiment of the present invention.

DETAILED DESCRIPTION

A Flash memory device with reduced power consumption and minimal erasevoltage disturb is obtained by forming all memory blocks in differentwell sectors. Each well sector can include for example, a device wellwithin which the NAND cell strings of the memory block(s) are formed in,and isolation wells for isolating the substrate from the device wells.At least one well sector is selectively coupled to an erase voltageduring an erase operation, such that unselected well sectors areinhibited from receiving the erase voltage, thereby minimizing erasedisturb in the unselected well sectors. Because each well sector has asmall area relative to a single well that includes all memory banks, thecapacitance of each well sector is small. This results in severaladvantages such as higher speed erasing or reduced charge pump size, aswill be discussed with reference to the following embodiments andexamples.

FIG. 6 illustrates a NAND flash memory bank according to an embodimentof the present invention. The particular example illustrated in FIG. 6has reduced substrate capacitance. FIG. 6 is a block diagram of one NANDflash memory bank 100, including a memory array 102 having NAND cellstrings similar to those shown in FIG. 2, a row decoder 104 for drivingwordlines connected to the devices of the NAND cell strings, and a pagebuffer 106 connected to bitlines for coupling read and program data tothe NAND cell strings. The memory array 102 includes at least twodifferent well sectors where NAND cell strings are formed. The NAND cellstrings are organized as memory blocks, such as those shown in FIG. 2,and each well sector includes at least one memory block. A charge pump108 generates the erase voltage Verase, which is provided to a selector110. Selector 110 selectively passes Verase to one of the at least twowell sectors of memory array 102.

Row decoder 104 receives a block address for selecting a particularmemory block for read, program and erase operations. Row decoder 104further receives a multi-bit row address RA for providing individual rowdrive signals that are used for activating selected wordlines, stringselect lines SSL and source select lines GSL. Row address RA can be apredecoded row address or a row address provided from the addressregisters, or any other upstream circuit. In the presently shownexample, a block address B_ADDR[1:m] is used to address up to 2^(m)memory blocks, where “m” can be any non zero integer value representingthe number of individual address signals that make up B_ADDR[1:m].Depending on the number of well sectors formed within memory array 102,one or all individual address signals of B_ADDR[1:m] are provided toselector 110. According to the present examples, the well sectorincluding the memory block selected for erasure will be biased toVerase. All unselected well sectors are inhibited from receiving Verase,by biasing them to 0V or VSS for example.

FIG. 7A shows an example of row decoder 104 of NAND flash memory bank100 shown in FIG. 6. Referring to FIGS. 6 and 7A, row decoder 104includes row decode logic 120, and memory block drive circuits 122 and124 for respective memory blocks. In the particular example, there arefour memory blocks in memory array 102. Row decode logic 120 receivesmulti-bit row address RA for generating individual row drive signalssuch as SS, S[1:i] and GS. In the present example, there is one rowdecode logic 120 for all the memory blocks in memory array 102, and therow drive signals SS, S[1:i] and GS are global signals provided to eachmemory block drive circuit 122 and 124 of the memory array 102. Inparticular, global signals SS, GS and S[1:1] correspond respectively toSSL, GSL and wordlines in each memory block drive circuit, such ascircuits 122 to 124 for example. Row decode logic 120 includes wellknown logic circuits for decoding multi-bit row address RA and itsdetails are not described. FIG. 7A shows two memory block drive circuits122 and 124, but those skilled in the art will understand that there isone memory block drive circuit for each memory block in memory array102. In the present example, there are a total of four memory blockdrive circuits, two of which are not shown in order to simplify thedrawing.

Memory block drive circuit 122 includes the same circuit elements asmemory block drive circuit 124, hence only the elements for memory blockdrive circuit 122 are described in further detail. Memory block drivecircuit 122 includes a block decoder 126 and row drivers 128. In theexample shown in FIG. 7A, block decoder 126 of memory block drivecircuit 122 receives a two-bit block addresses B_ADDR[1:2] for enablingits corresponding row drivers 128. Accordingly, one block decoder isenabled for any combination of B_ADDR[1:2] to select the memory blockfor an erase, program or read operation. Row drivers 128 includesdevices for passing row drive signals SS, S[1:i] and GS to SSL, thewordlines WL[1:i] and GSL, respectively, of the NAND cell strings in thememory block. Therefore in response to the multi-bit row address RA,SSL, GSL and one wordline WL1 to WLi, are driven to the active logiclevel such as VDD by row decode logic 120. In response to a blockaddress, only the row drivers 128 of one selected memory block addressedby block addresses B_ADDR[1:2] are enabled for driving or passing therow drive signals SS, S[1:i] and GS as SSL, WL[1:i] and GSL respectivelyto the NAND cell strings. In an unselected memory block the row drivers128 are disabled, thereby preventing SSL, WL[1:i] and GSL from receivingthe voltage level of row drive signals SS, S[1:i] and GS respectively.

FIG. 7B is circuit schematic of the block decoder 126 and row drivers128 of one memory block drive circuit, such as memory block drivecircuit 122, shown in FIG. 7A. Block decoder 126 is associated with onememory block, and includes a cross coupled inverter latch circuit andcharge pump. The latch circuit includes cross-coupled inverters 130 and132, an n-channel reset transistor 134, and n-channel enable transistors136 and 138. The latch circuit is enabled, or set, when latch enablesignal LTCH_EN and a decoded block address BA[1:4] are at the high logiclevel. Decoded block address BA[1:4] is also referred to as a blockselect signal. The four individual signals of decoded block addressBA[1:4] are generated by AND logic gate 140, which receives blockaddresses B_ADDR[1] and B_ADDR[2]. Those skilled in the art shouldunderstand that the AND logic gate 140 for different block decoders 126of the memory bank receives is responsive to different logic statecombinations of B_ADDR[1] and B_ADDR[2] for selecting one memory blockfor read, program and erase operations. When a reset signal RST_BD isdriven to the high logic level, of VDD for example, reset transistor 134is turned on to couple the input of inverter 132 to VSS. This results inthe latch circuit of inverters 130 and 132 to be reset.

The block decoder 126 includes a local charge pump coupled to the outputof inverter 130. The charge pump includes a depletion mode n-channelpass transistor 142, a native n-channel diode-connected boost transistor144, a high breakdown voltage n-channel decoupling transistor 146, ahigh breakdown voltage n-channel clamp transistor 148, a NAND logic gate150, and a capacitor 152. NAND logic gate 150 has one input terminalcoupled to the output of inverter 130 and another input terminal forreceiving controlled signal OSC, for driving one terminal of capacitor152. Pass transistor 142 is controlled by the complement of a programsignal PGM, referred to as PGMb. The common terminals of decouplingtransistor 146 and clamp transistor 148 are coupled to high voltage VH.

The operation of the charge pump is now described. During a read orerase operation, PGMb is at the high logic level and OSC is maintainedat the low logic level. Therefore, circuit elements 152, 144, 146 and148 are inactive, and the output terminal BD_OUT reflects the logiclevel appearing on the output of inverter 130. During a programoperation, PGMb is at the low logic level, and OSC is allowed tooscillate between the high and low logic levels at a predeterminedfrequency. If the output of inverter 130 is at the high logic level,then capacitor 152 will repeatedly accumulate charge on its otherterminal and discharge the accumulated charge through boost transistor144. Decoupling transistor 146 isolates VH from the boosted voltage onthe gate of boost transistor 144. Clamp transistor 148 maintains thevoltage level of output terminal BD_OUT at about VH+Vtn, where Vtn isthe threshold voltage of clamp transistor 148. The local charge pumpshown in FIG. 7B is one example circuit which can be used to drivesignals to a voltage levels higher than the supply voltage VDD, butpersons skilled in the art will understand other charge pump circuitscan be used with equal effectiveness.

Row drivers 128 includes a plurality of n-channel pass transistors 154,each having its gate terminal electrically coupled to the outputterminal BD_OUT for passing the row drive signals SS, S[1:i] and GS atSSL, the wordlines WL[1:i] and GSL, respectively, to the NAND cellstrings. If the output terminal BD_OUT is driven above VDD, then rowdrive signals SS, S[1:i] and GS greater than VDD can be passed onto SSL,WL[1:i] and GSL lines, respectively. If the output terminal BD_OUT is atVSS, then the pass transistors 154 will be turned off to decouple therow drive signals SS, S[1:i] and GS from SSL, WL[1:i] and GSL,respectively.

FIG. 8A shows a NAND flash memory bank according to an example of theNAND flash memory bank embodiment of FIG. 6. The NAND flash memory bankhas reduced substrate capacitance. Referring to FIG. 8A, one NAND flashmemory bank 200 has a memory array 202 including NAND cell stringssimilar to those shown in FIG. 2. The NAND flash memory bank 200 hasalso a row decoder 204 for driving wordlines connected to the devices ofthe NAND cell strings, and a page buffer 206 connected to bitlines forcoupling read and program data to the NAND cell strings. A charge pump208 generates the erase voltage Verase, which is provided to a selector210. In the illustrated example, the memory array 202 has four differentwell sectors, each selectively receiving Verase during a block eraseoperation. Each of the four well sectors includes exactly one memoryblock, referred to as Block[1], Block[2], Block[3] and Block[4].Accordingly, a two-bit block address B_ADDR[1:2] is used for selectingone of four memory blocks, and row decoder 204 includes the appropriatelogic for decoding the two-bit block address B_ADDR[1:2]. The same blockaddress is received by selector 210 for passing Verase to the wellsector which includes the corresponding selected memory block. Selector210 can be implemented as a 1 to 4 demultiplexor responsive to a two-bitcontrol or address signal.

FIG. 8B is a cross-sectional diagram of memory array 202 taken alongline B-B′ in FIG. 8A, and angled to show specific features on itssurface. In FIG. 8B, only the first three well sectors 220, 222 and 224are shown, where each well sector includes a p-type well 226 formedwithin an n-type isolation well 228, the n-type isolation well 228 beingformed within a p-type substrate 230. Both the n-type isolation well 228and the p-type well 226 are ion implanted regions having two-dimensionalsurface areas on the chip formed through well known masking steps thatdelineate their specific shapes. Because the substrate 230 is p-type,the n-type isolation wells 228 are used to electrically isolate thep-type substrate 230 from the p-type wells 226. The depth andconcentration of wells 226 and 228 is determined by the ion implantationenergy and dose, which are both fabrication design parameters of thesemiconductor device. As clearly shown in FIG. 8B, the NAND cell stringsare formed in the p-type wells 226. In an alternate example wheresubstrate 230 is n-type instead of p-type, the n-type isolation wells228 are omitted, and the well sectors 220, 222 and 224 have p-type wells226. According to the present examples, each well sector includes atleast the device well the NAND cell strings are formed within, andoptionally includes isolation wells for isolating the substrate 230 fromthe device wells 226.

In the presently shown example of memory array 202, well sectors 220,222 and 224 include memory blocks 234, 236 and 238 respectively. Eachmemory block includes NAND cell strings 232 electrically coupled torespective bitlines, such as bitlines BLk and BLk+1 to BLj for example.While not shown in FIG. 8B, the bitlines are connected to a page bufferfor sensing cell data and for providing program data. The possiblecircuit implementations of page buffers, and their operation should bewell known to persons of skill in the art. The selector 210 is shown inFIG. 8B to show its interconnection with the well sectors 220, 222 and224 of memory array 202. Selector 210 receives Verase and electricallycouples Verase to one of the four well sectors in response to thetwo-bit block address B_ADDR[1:2]. As shown in FIG. 8B, each output ofselector 210 is connected to a respective p-type well 226 as well as itscorresponding isolation well 228. This is to ensure that the junctionbetween the p-type substrate 230 and the n-type isolation well 228 isreverse biased.

The advantage of having separate well sectors for each memory block ofthe memory bank is the reduced capacitive loading of the charge pumprelative to the single well memory bank of the known NAND flash memorybank of FIG. 3. An example comparison between the single well memorybank of FIG. 3 and the multiple well sector example of FIGS. 8A and 8Bfollows. It is first assumed that the memory bank of FIG. 3 and thememory bank of FIG. 8A include exactly four memory blocks, have the samenumber of NAND cell strings per memory block, and the same charge pumpfabricated with the same process and technology node. As previouslydiscussed, t_delay is the amount of time p-type well 54 charges up fromVSS to Verase during an erase operation for erasing one memory block. Inthe NAND flash memory bank of FIGS. 8A/8B, the capacitance of one p-typewell 226 is effectively ¼ that of p-type well 54. Accordingly, thevoltage level of p-type well 226 will rise from VSS to Verase in lesstime than t_delay. Alternatively, if the erase time of the memory bankof FIG. 3 is to be maintained for the NAND flash memory bank of FIG. 8A,then the charge pump circuit capacity can be decreased. This can be donethrough a combination of reducing the size of capacitor elements and/oreliminating entire capacitor elements. Therefore semiconductor chip areais reduced, and the cost of the NAND flash memory device iscorrespondingly reduced. Furthermore, since the capacitance of each wellsector is smaller than that of p-type well 54, significant power savingsis realized.

A further advantage of the NAND flash memory bank of FIGS. 8A and 8B isthat unselected memory blocks do not receive Verase, thereby eliminatingerase disturb in the unselected memory blocks. This is because eachmemory block of FIGS. 8A and 8B resides in its own well sector.Accordingly, the wordlines in the unselected memory blocks residing inunselected well sectors do not self-boost, and are allowed to float atabout the VSS voltage level.

An erase operation executed upon the example NAND flash memory bank ofFIGS. 8A and 8B is now described with reference to the memory blockdrive circuit 122 shown in FIG. 7B. In the erase operation for theexample of FIGS. 8A and 8B, one memory block is selected while otherblocks remain unselected. In other words, one memory block is enabledwhile remaining memory blocks are disabled. In the present example eraseoperation, it is assumed that only memory block Block[1] is to beerased. Accordingly, the operation of memory block drive circuit 122 isdescribed, while the operation of memory block drive circuit 124 isdescribed for any one of the unselected memory blocks. To select amemory block for erasure, LTCH_EN and BA[1:4] of memory block drivecircuit 122 will be at the high logic level, thereby setting the levelshifter circuit to output high voltage Vh. Therefore, all the passtransistors 154 of wordline driver circuit 128 are turned on. The rowdrive signals S1 to Si are driven to VSS while row drive signals SS andGS are floated, and the selected well sector having the selected memoryblock is biased to Verase. Memory block drive circuit 124 for anunselected memory block will have its corresponding block decodercircuit output set to output low voltage Vn. Therefore, all the passtransistors 154 corresponding to the unselected memory blocks will beturned off. Accordingly, the wordlines, SSL and GSL for the unselectedmemory blocks will float at approximately VSS, since these lines aretypically biased to VSS after any read or program operation.

Table 1 below summarizes example biasing conditions during an eraseoperation for a selected memory block and an unselected memory block,where the selected memory block resides in one well sector and theunselected memory block resides in a different well sector. A memoryblock is selected by providing the block address B_ADDR[1:2] of theselected memory block to row decoder 204, which is decoded to enablecontrol of the wordlines and select lines (SSL and GSL) corresponding tothe selected memory block. Because B_ADDR[1:2] is received by selector210, Verase is passed to the well sector which includes the selectedmemory block.

TABLE 1 Unselected Well Sector Selected Well Sector Unselected MemorySelected Memory Block Block Bitlines (B/L) Clamped to Clamped to Verase−0.6 V Verase −0.6 V String Select Line Floated Unselected (SSL)Wordlines 0 V Unselected Ground Select Line Floated Unselected (GSL)Common Source Line Clamped to 0 V (CSL) Verase −0.6 V P-Well Verase 0 V

In order to erase a selected memory block in the example NAND flashmemory bank of FIGS. 8A and 8B, the wordlines are biased to VSS, or 0V,the common source line CSL (hereinafter referred to as “CSL line”) isclamped to about Verase-0.6V, and SSL and GSL are left to float. Decodedrow drive signals SS and GS for the SSL and GSL lines are floated duringerase to minimize the electric field on pass transistors for SSL andGSL. It is noted that the capacitance of drive lines SS and GS are muchlarger than that of SSL and GSL. Therefore, SSL and GSL may remain atnearly VSS because any boosted charge on SSL and GSL resulting from thewell voltage rising to Verase will leak through the pass transistors tothe SS and GS drive lines.

Finally, the selected well sector within which the selected memory blockresides is biased to Verase. Under these conditions, trapped charge inthe floating gates of the flash memory cells of the NAND cell stringswill emit their charge to the well. As previously shown in FIG. 2 andFIG. 4, all bitlines are shared by the memory blocks in the memory bank,and have bitline contacts electrically connected to the n+ diffusionregion 88 corresponding to each NAND cell string. The n+ diffusionregions 88 are forward biased when the selected well sector is raised tothe erase voltage Verase, resulting in the bitlines being clamped toVerase-0.6V. As previously shown in FIG. 2 and FIG. 4, the CSL line isshared by all the NAND cell strings in the same memory block via n+diffusion region 82. Hence when the selected well sector is raised toVerase, the n+ diffusion region 82 is forward biased raise and clamp theCSL line of the selected memory block in the selected well sector toVerase-0.6V. On the other hand, the CSL line for an unselected memoryblock in an unselected well sector is biased to VSS or 0V. It is notedthat in the present examples, the CSL line is common only to the NANDcell strings of one memory block. All wordlines, string select lines(SSL) and ground select lines (GSL) in the unselected memory blockremain in an unselected state, meaning that the pass transistors 154 inthe wordline driver 128 are turned off.

FIGS. 8A and 8B illustrate an example NAND flash memory bank where thereis exactly one memory block per well sector in memory bank array 202.Depending on the fabrication process and technology node being used formanufacturing NAND flash memory bank 200, adjacent well sectors arespaced from each other by a minimum distance “D”, as shown in FIG. 8B.This minimum distance D can be set to be the minimum design rule spacingfor adjacent n-type isolation wells. By example only, the spacingbetween adjacent n-type isolation wells 228 can be between 3 to 10microns.

In another example of memory array 202, each well sector of the memorybank includes more than one memory block to minimize the size of thememory array while reducing the capacitance of each well sector. FIG. 9Ais a block diagram of a NAND flash memory bank with reduced memory arrayarea relative to the example NAND flash memory bank of FIG. 8A.Referring to FIG. 9A, one NAND flash memory bank 300 has a memory array302 including NAND cell strings similar to those shown in FIG. 2, a rowdecoder 304 for driving wordlines connected to the devices of the NANDcell strings, and a page buffer 306 connected to bitlines for couplingread and program data to the NAND cell strings. A charge pump 308generates the erase voltage Verase, which is provided to a selector 310.By example only, the illustrated NAND flash memory bank has a memoryarray 302 including two different well sectors, each selectivelyreceiving Verase during a block erase operation. Memory array 302includes four memory blocks, referred to as Block[1], Block[2], Block[3]and Block[4]. Accordingly, a two-bit block address B_ADDR[1:2] is usedfor selecting one of four memory blocks and row decoder 304 includes theappropriate logic for decoding the two-bit block address BADDR[1:2].

In the memory array 302, a first well sector includes memory blocksBlock[1] and Block[2], and a second well sector includes memory blocksBlock[3] and Block[4]. A pair of two well sectors is selected during anerase operation. A single bit block address signal is received byselector 310 for passing Verase to the well sector which includes theselected memory block. If B_ADDR[1] is the most significant blockaddress bit for selecting which pair of memory blocks is to be selected,then B_ADDR[2] is the least significant block address bit for selectingone memory block of the selected pair of memory blocks. Selector 310 isa 1-to-2 demultiplexor, or selector, responsive to a single-bit controlor address signal. Because the example of FIG. 9A has two memory blocksformed in each well sector, only one of the two well sectors is selectedfor any memory block erase operation. Therefore block address B_ADDR[1]is used by selector 310 for passing Verase to one of the two wellsectors. In other words, selector 310 receives a portion of the blockaddress for selectively passing Verase to one of the two well sectors.

FIG. 9B is a cross-sectional diagram of memory array 302 of FIG. 9Ataken along line C-C′, and angled to show specific features on itssurface. In FIG. 9B, two well sectors 320 and 322 are shown, where eachwell sector includes a p-type well 324 formed within an n-type isolationwell 326, the n-type isolation well 326 being formed within a p-typesubstrate 328. Both the n-type isolation well 326 and the p-type well324 are ion implanted regions having two-dimensional surface areas onthe chip formed through well known masking steps that delineate theirspecific shapes. The depth and concentration of wells 226 and 228 isdetermined by the ion implantation energy and dose, which are bothfabrication design parameters of the semiconductor device. As clearlyshown in FIG. 9B, the NAND cell strings are formed in the p-type wells324. In an alternate example where substrate 328 is n-type instead ofp-type, the n-type isolation wells 326 are omitted, and the well sectors320 and 322 have p-type wells 324. According to the present examples,each well sector includes at least the device well the NAND cell stringsare formed within, and optionally includes isolation wells for isolatingthe substrate from the device wells.

In the presently shown example of memory array 302, well sector 320includes memory blocks 330 and 332, which correspond to memory blocksBlock[1] and Block[2] respectively. Well sector 322 includes memoryblocks 334 and 336, which correspond to memory blocks Block[3] andBlock[4] respectively. Each memory block includes NAND cell strings 338electrically coupled to respective bitlines, such as bitlines BLk andBLk+1 to BLj for example. While not shown in FIG. 9B, the bitlines areconnected to a page buffer for sensing cell data and for providingprogram data. The possible circuit implementation of page buffers, andtheir operation should be well known to persons of skill in the art. Theselector 310 is shown in FIG. 9B to show its interconnection with thewell sectors 320 and 322 of memory array 302. Selector 310 receivesVerase and electrically couples Verase to one of the two well sectors inresponse to the single-bit block address B_ADDR[1]. As shown in FIG. 9B,each output of selector 310 is connected to a respective p-type well 324as well as its corresponding isolation well 326.

The erase operation for a selected memory block in the example NANDflash memory bank of FIGS. 9A and 9B is similar to that described forthe example NAND flash memory bank of FIGS. 8A and 8B, except that anerase inhibit scheme is applied to the unselected memory block of theselected well sector that receives the erase voltage Verase. This is dueto the fact that each well sector has both a selected memory block to beerased and an unselected memory block. Therefore, the flash memory cellsof the unselected memory block in the well sector receiving Verase iserase inhibited. By example, the previously described self-boostingerase inhibit scheme can be used for inhibiting erasure of the flashmemory cells of the unselected memory block in the selected well sector,where a selected well sector is the one that receives Verase. Forexample, if memory block 332 (Block[2]) is selected for erasure, thenthe wordlines and select lines (SSL and GSL) corresponding to selectedmemory block 332 are biased to the erase condition, and Verase isapplied to well sector 320. Memory block 330 is erase inhibited toprevent erasure of its flash memory cells because it is formed withinthe same p-type well 324 as memory block 332.

Table 2 summarizes example biasing conditions during an erase operationfor a selected memory block and an unselected memory block in theselected well sector that receives Verase, and for unselected memoryblocks in an unselected well sector that does not receive Verase.

TABLE 2 Selected well sector Unselected well Selected Memory Unselectedsector Block Memory Blocks All Memory Blocks Bitlines (BL) Clamped toClamped to Clamped to Verase −0.6 V Verase −0.6 V Verase −0.6 V StringSelect Line Floated Boosted to about Unselected (SSL) 90% of VeraseWordlines (WL0- 0 V Boosted to about Unselected WL31) 90% of VeraseGround Select Line Floated Boosted to about Unselected (GSL) 90% ofVerase Common Source Line Clamped to Clamped to 0 V (CSL) Verase −0.6 VVerase −0.6 V Well sector Verase Verase 0 V

The erase bias conditions for the word lines and SSL and GSL are thesame as for the NAND flash memory bank examples of FIGS. 8A, 8B andFIGS. 9A, 9B, as are the bias conditions for the unselected memoryblocks in the unselected well sectors. However, for the unselectedmemory blocks of the selected well sector, the wordlines are selfboosted to about Verase. When the wordlines are approximately Verase,there is a minimal electrical field formed between the wordlines and thep-type well 324 of the selected well sector, thereby inhibiting erasureof the flash memory cells of the unselected memory block.

While FIGS. 8A, 8B and 9A, 9B show examples where there is exactly onememory block formed per well sector and two memory blocks formed perwell sector in one memory array, alternate examples can include anynumber of well sectors in each memory array, where each well sector caninclude any number of memory blocks formed therein.

In both the example NAND flash memory banks of FIGS. 8A, 8B and 9A, 9B,the voltage of the bitlines rises to about Verase-0.6V when the p-typewell of the well sector rises to Verase. With reference to FIG. 4 forexample, the p-type well 54 and the n+ diffusion region 88 has a p-njunction that is forward biased when Verase is applied. Accordingly,bitline 90 will clamp to about Verase-0.6V, where 0.6V is the forwardbias voltage drop across the p-n junction. Each bitline is electricallyconnected to the NAND cell strings in each memory block of the memoryarray. Accordingly, this clamped voltage of the bitline is applied tothe n+ diffusion regions 88 of all the NAND cell strings of the column,and notably to the n+ diffusion regions 88 of the NAND cell strings inthe unselected well sectors. Since the unselected well sectors arebiased to VSS, or float near VSS, the bitlines potentially provide acharge leakage path between the selected well sector and one or moreunselected well sectors. More specifically, Verase being applied to theselected well sector can be discharged to VSS through the unselectedwell sector if junction breakdown occurs at the p-n junctions of the n+diffusion region 88 and the p-type well 54 of each NAND cell string.This can delay the rise of Verase in the selected well sector, or evendisrupt the erase process if the selected well sector voltage neverfully reaches Verase. With this understanding of the bitline voltageduring erase operations, the junction breakdown voltage of the n+diffusion region 88 is engineered to withstand breakdown when thebitline rises to Verase-0.6V.

Although junction breakdown engineering is a possible solution to thisproblem, a simpler solution is to electrically isolate the bitlineconnected to the selected well sector from the unselected well sectors.According to a present embodiment, since well sectors are spaced fromeach other due to design rules, an isolation device is included in-linewith the bitline between well sectors. FIG. 10 illustrates an example ofthis embodiment.

FIG. 10 is another example of the memory array 302 shown in FIGS. 9A and9B. In FIG. 10, memory array 400 has the same elements as those shown inFIG. 9B. In order to isolate the bitlines connected to one well sectorfrom the NAND cell strings of another well sector, isolation devices,such as n-channel transistors 402 are formed in the space between wellsectors. The n-channel transistors 402 can be formed as high voltagetransistor devices, at the same time other high voltage transistordevices are formed on the memory device. The bitlines electricallyconnected to the NAND cell strings of one well sector are connected toone terminal of isolation devices 402, while the bitlines electricallyconnected to the NAND cell strings of an adjacent well sector areconnected to the other terminal of isolation devices 402. Therefore, thebitlines connected to the NAND cell strings in one well sector arereferred to as bitline segments. In FIG. 10, these bitline segments areindicated by reference numbers 404. If there are more than two wellsectors in memory array 400, then there are additional isolation devices402 connected in-line or in series, with each bitline segment 404. Thegate terminals of all the isolation devices 400 receive a bias voltageViso, which is selected to be at least a voltage level sufficiently highfor passing the highest voltage level that is applied to the bitlineduring read and program operations. The isolation devices are henceenabled during a read or program operation. In other words, the gateterminals of all the isolation devices can be overdriven to a levelabove the supply voltage VDD. There may be devices where overdriving thegate terminals is not necessary, and it is sufficient to drive them tothe supply voltage VDD.

For example, if VDD is applied to bitlines during a programmingoperation for programming a particular logic state to a flash memorycell, then the bias voltage Viso should be at least VDD+Vtn, where Vtnis a threshold voltage of an n-channel transistor. By setting Viso tosuch a voltage level, a full VDD voltage level is maintained in allbitline segments during programming. During an erase operation for amemory block in a selected well sector, the corresponding bitlinesegment will rise to about Verase-0.6V. But because the isolationdevices 404 have their gates biased to Viso=VDD+Vtn, the other bitlinesegments are limited to being charged to VDD. Therefore, minimaladditional junction breakdown engineering is required since the n+diffusion region connected to the bitlines is already designed towithstand a VDD voltage level.

In one example of the memory array 400, Viso is statically maintained atthe same voltage level during read, program and erase operations.Persons skilled in the art should understand that there are differentways to provide Viso. In an alternate example Viso can be a decodedsignal, meaning that Viso is selectively applied to the gates of theisolation devices adjacent to the selected well sector.

FIG. 11 is a circuit schematic of an example of the embodiment of FIG.10. In FIG. 11, one memory bank has a memory array with dynamicallyactivated isolation devices formed between well sectors for definingbitline segments, and a bitline segment decoder for selectivelyisolating one bitline segment from the other bitline segments in eachcolumn. The row decoder is not shown in order to simplify the schematic,but those skilled in the art will understand that they are necessary fordriving the wordlines of the NAND cell strings in each memory block. Amemory bank 500 includes a memory array 502, a page buffer 504 and abitline segment decoder 506. Memory array 502 of the present exampleincludes four well sectors 508, 510, 512 and 514, where each well sectorincludes exactly one memory block. Accordingly, memory array 502 has thesame structure as memory array 202 of FIGS. 8A/8B. Formed between eachwell sector are isolation devices 516, each being an n-channeltransistor for the present example. Bitline segments 518 are connectedto the NAND cell strings in each memory block, and are connected to anisolation device at both ends. Bitline tail segments 520 and 522 arebitline segments which are not connected to any NAND cell strings andare connected to one isolation device 516 and some other terminatingcircuit. For example, bitline tail segments 520 are connected toisolation devices 516 above well sector 508 and to a bitline prechargecircuit 524. Bitline tail segments 522 on the other hand are connectedto isolation devices 516 below well sector 514 and to page buffer 504.While the present example of FIG. 11 shows each well sector having onememory block formed therein, according to alternate examples each wellsector can have multiple memory blocks formed therein, where a bitlinesegment is commonly connected to the multiple memory blocks residing inthe same well sector. The isolation devices 516 driven by NAND logicgate 532 and NAND logic gate 540 are optional, meaning that the bitlinesegments 518 corresponding to well sectors 508 and 514 can extend to theprecharge circuit 524 and the page buffer 504 respectively.

Bitline segment decoder 506 is responsible for disabling, renderingelectrically non-conductive, or turning off, the selected isolationdevices 516 for isolating a bitline segment 518 corresponding to aselected well sector including the selected memory block to be erased,from all other bitline segments 518. Bitline segment decoder 506includes OR logic gates 526, 528 and 530, and NAND logic gates 532, 534,536, 538 and 540. Each of OR logic gates 526, 528 and 530 receives twodifferent well sector selection signals, and more specifically, two wellsector selection signals corresponding to adjacent well sectors. Becauseadjacent well sectors share the same isolation device 516, an OR logicgate is used to disable the isolation devices 516 between two adjacentwell sectors when either is selected or addressed for an eraseoperation. In the example of FIG. 11, the well sector selection signalscorrespond to decoded block address BA[1:4] because there is exactly onememory block in each well sector. The number of well sector selectionsignals required by bitline segment decoder 506 depends on the number ofwell sectors in the memory array of the memory bank. For example, ifthere were exactly two memory blocks in each well sector of FIG. 11,then there are a total of eight (8) memory blocks. Accordingly, threeblock address signals are used for individually selecting one of eightmemory blocks, but two of the most significant block address signals canbe used for generating the well sector selection signals. For example, amodified version of the memory block drive circuit 122 of FIG. 7B canhave a three input AND logic gate instead of the 2 input AND logic gate140 for decoding B_ADDR[1], B_ADDR[2] and B_ADDR[3] to perform a one ofeight selection. Accordingly, persons skilled in the art will understandthat there would be a total of eight memory block drive circuits, onefor each memory block. In the present example, the two most significantblock addresses B_ADDR[3] and B_ADDR[2] are decoded using well knownlogic to provide four well sector selection signals.

NAND logic gates 534, 536 and 538 each have a first input for receivingthe output of OR logic gates 526, 528 and 530 respectively. NAND logicgate 532 has a first input for receiving a well sector selection signaldirectly because the isolation devices 516 above well sector 508 are notshared with another well sector. Similarly, NAND logic gate 540 has afirst input for receiving a well sector selection signal directlybecause the isolation devices 516 below well sector 514 are not sharedwith another well sector. The second input of all the NAND logic gatesreceives an erase signal ERASE, and the output of each NAND logic gatedrives a set of isolation devices 516 adjacent to at least one wellsector. The high logic level output of each NAND logic gate is set suchthat the isolation devices 516 are driven to a voltage level sufficientto allow the maximum bitline voltage level to pass through it. Forexample, if the maximum bitline voltage is VDD then the NAND logic gatesare supplied with a positive voltage higher than VDD.

According to the present example, signal ERASE is set to the low logiclevel for a read or program operation, thereby turning on or overdrivingall the isolation devices 516. As previously stated all the NAND logicgates can be supplied with a voltage level greater than the VDD supplyvoltage. Therefore the logic states of the well sector selection signalsBA[1:4] are ignored. During an erase operation, signal ERASE is set tothe high logic level. Now NAND logic gates 532 and 540 are responsive tothe well sector selection signals BA[1:4], and NAND logic gates 534, 536and 538 are responsive to an output of a corresponding OR logic gate.The NAND logic gates are responsive by driving their outputs to theinactive logic level when their first and second inputs are both at thehigh logic level. Accordingly, when one well sector selection signal isat the active high logic level, the sets of isolation devices adjacentto the correspondingly selected well sector are turned off. Then theerase voltage Verase is applied to the selected well sector.

For example, if a memory block in well sector 510 is to be erased, thenonly address BA[2] is driven to the high logic level. Then NAND logicgates 534 and 536 drive their respective outputs to the low logic level,such as VSS, and the isolation devices 516 having their gateselectrically coupled to the outputs of NAND logic gates 534 and 536 willturn off. Therefore, the bitline segments 518 of well sector 510 areisolated from the other bitline segments of the other well sectors.

In the example of FIG. 11, bitline segments 518 are selectivelydisconnected from the other bitline segments in response to decodedblock addresses BA[1:4] during an erase operation. In an alternateexample, all the isolation devices are globally enabled and disabled inresponse to the operating mode of the memory device. More specifically,if an erase operation is executed, then all the isolation devices 516are turned off independently of any address information.

FIG. 12 is a circuit schematic of one memory bank having a memory arraywith dynamically activated isolation devices formed between wellsectors, according to an alternate example of the embodiment of FIG. 10.In FIG. 12, memory bank 550 includes the same memory array 502 shown inFIG. 11, but now bitline segment decoder 506 is replaced with a simplesegment decoupling logic 552. Segment decoupling logic 552 includesinverters 554, 556, 558, 560 and 562, each of which receives the erasesignal ERASE in parallel, and drives a respective set of isolationdevices 516. In the present example, the inverters are provided with asupply voltage VDD or a voltage level greater than VDD to overdrive theisolation devices 516 during non-erase operations. In an eraseoperation, signal ERASE is driven to the high logic level, and all theinverters drive the gates of the isolation devices 516 to VSS. All theisolation devices 516 turn off, resulting in all the bitline segments518 being isolated from each other, and Verase is applied to theselected well sector that includes the memory block to be erased. In aprogram or read operation, ERASE is at the low logic level, and theisolation devices have their gates driven to a voltage level of at leastVDD.

In both the examples of FIGS. 11 and 12, ERASE is at the inactive lowlogic level during a read or program operation to ensure that all theisolation devices 516 are at least turned on, or overdriven.Unfortunately, a known issue affecting sensing time is bitlinecapacitance. Those skilled in the art understand that as the bitlinelength increases, its capacitance increases as well. Because the currentthrough a conducting flash memory cell is small, this current isdifficult to sense when the bitline capacitance is high. Therefore, theisolation devices shown in FIGS. 11 and 12 can be used in an alternateembodiment of the invention to reduce bitline capacitance to shortensensing time.

FIG. 13 is a circuit schematic of an alternate memory bank 570 accordingto an example of the embodiment for reducing bitline capacitance toshorten sensing time. Memory array 502 is the same as the one shown inFIG. 11, but an alternate bitline segment decoder 572 according to anexample of the present embodiment is illustrated. Bitline segmentdecoder 572 turns off all the isolation devices 516 in memory array 502during an erase operation, and selectively turns off specific isolationdevices 516 during a read operation. In FIG. 13, bitline segment decoder572 includes an inverter 574 having its output for providing anisolation drive signal coupled to a respective set of isolation devices516 between the bitline precharge circuit 524 and well sector 508, andOR logic gates 576, 578, 580 and 582 each having an output for providingan isolation drive signal coupled to respective sets of isolationdevices 516. A first input of OR logic gates 576, 578, 580 and 582 iscoupled to the outputs of NOR logic gates 584, 586, 588 and 590respectively. Inverter 574 and each pairing of a NOR logic gate and ORlogic gate are referred to as an isolation device driver, having outputsfor driving respective isolation devices. A second input of OR logicgates 578, 580 and 582 receives the output of another isolation devicedriver, while the second input of OR logic gate 576 is grounded. Eachisolation device driver provides its isolation drive signal for one setof isolation devices 516 to one isolation device driver that providesits isolation drive signal to a second set of isolation devices 516,where the second set of isolation devices is located between the firstset of isolation devices and the page buffer 504. A first input of ORlogic gates 584, 586, 588 and 590 receives the erase signal ERASE. Asecond input of NOR logic gates 584, 586, 588 and 590 is an invertedinput, and receives well sector selection signals BA[1], BA[2], BA[3]and BA[4] respectively.

As previously mentioned for the example arrangement of memory array 502,each well sector includes one memory block, hence decoded blockaddresses BA[1:4] can be used as the well sector selection signals. Theoperation of bitline segment decoder 572 is straightforward during anerase operation. Signal ERASE is set to the active high logic level,thereby causing each NOR logic gate to provide a low logic level outputwhich is passed by each OR logic gate as the isolation drive signal toall the isolation devices 516. Therefore all the isolation devices 516are turned off while ERASE is at the high logic level, and Verase can beapplied to the selected well sector.

During a read operation, only the isolation devices 516 located betweenthe well sector containing the selected memory block and the page buffer504 are turned on, or overdriven. All other isolation devices are turnedoff to minimize the bitline capacitance seen by the selected NAND cellstrings of the selected memory block. For example, when a memory blockin well sector 512 is selected for a read operation, isolation devices516 between well sectors 512 and 514 are turned on, as are the isolationdevices 516 between well sector 514 and page buffer 504. Thereforeduring a read operation for a selected memory block, only the bitlinesegments 518 corresponding to the selected well sector including theselected memory block, and the bitline segments 518 corresponding to allthe other intervening well sectors between the selected well sector andthe page buffer are electrically coupled to each other. In the exampleof FIG. 13, each OR logic gate of the isolation device driver is anaddress overrider circuit. It is noted that the isolation device driverhaving inverter 574 does not include a NOR logic gate and an OR logicgate, since there are no further well sectors between the isolationdevices 516 it controls and bitline precharge circuit 524. The addressoverrider circuit allows for an isolation device driver activated by awell sector selection signal to enable or activate the next set ofisolation devices 516 proximate to the page buffer 504.

In an example read operation the selected memory block resides in wellsector 510, thus BA[2] is driven to the high logic level and ERASE is atthe low logic level. With ERASE at the low logic level, inverter 574turns on or overdrives its isolation devices. NOR gate 584 provides alow logic level output since BA[1] is at the low logic level, which ispassed by OR gate 576 to its respective set of isolation devices 516.NOR gate 586 provides a high logic level output to OR gate 578, whichalso receives the low logic level output from OR gate 576. Accordingly,the isolation devices 516 between well sectors 510 and 512 are turned onor overdriven. With BA[3] at the low logic level, NOR gate 588 drivesits output to the low logic level. However, the high logic level outputof OR gate 578 is received by OR gate 580. Therefore the output of NORgate 588 is overridden. Similarly, the output or NOR gate 590 isoverridden by OR gate 582, thus the isolation devices 516 between wellsectors 512 and 514 and page buffer 504 are turned on.

With the reading scheme shown in FIG. 13, the worst case read situationoccurs when the selected memory block resides within well sector 508,which is the furthest from page buffer 504. On the other hand, the bestcase read situation occurs when the selected memory block resides withinwell sector 514, which is the closest to page buffer 504. Therefore, thesense timing and data output transfer timing can be adjusted based onthe selected well sector that includes the selected memory block to beread. In one example application, a specific number of the memory blocksformed in well sectors proximate to the page buffer 504 can bedesignated as high speed memory blocks. The remaining memory blocksresiding in the well sectors further from the page buffer 504 can bedesignated as regular speed memory blocks. While in use with an externalsystem, data can be selectively stored in either high speed or regularspeed memory blocks for high speed or regular speed read operations.

FIG. 14 is a flow chart summarizing the method by which a selectedmemory block is erased, in accordance with the previously describedembodiments of the invention. The following method is applicable to amemory array having any number of well sectors, where each well sectorincludes at least one memory bank. Starting at step 600 a selectedmemory block is biased for erasing, which is done by setting thewordlines, SSL and GSL to the appropriate erase bias conditions. Table 2illustrates example erase bias conditions for the memory block to beerased. If each well sector includes at least two memory blocks, thenthere is an unselected memory block in the same well sector as theselected memory block to be erased. Therefore at step 602, theunselected memory block in the same well sector as the selected memoryblock is biased to inhibit erase of its memory cells. Once again, Table2 illustrates example erase inhibit bias conditions for the wordlines,SSL and GSL. If on the other hand each well sector includes exactly onememory block, then step 602 is skipped. Proceeding to step 604, theerase voltage is applied to the well sector containing the selectedmemory block. Following at step 606, the bitline segments of theselected well sector are decoupled from the other bitline segmentseither at the same time or just after Verase is applied to the selectedwell sector. It is noted that it is sufficient to decouple the bitlinesegment of the selected well sector from the bitline segments ofadjacent well sectors. This decoupling is either dynamic decoupling orstatic decoupling. Static decoupling occurs when the isolation devicesare statically biased to Viso, and the bitline segments of the selectedwell sector self-decouple from the other bitline segments as its voltagerises past Viso. Dynamic decoupling occurs by actively turning off theisolation devices either globally in response to a single controlsignal, such as ERASE, or in response to a well sector selection signal.If dynamic decoupling is used, then the isolation devices can be turnedoff prior to application of Verase to the selected well sector.

The presently shown memory bank embodiments and examples of theinvention reduce power consumption while improving the speed at which amemory block is erased by forming well sectors that can be selectivelybiased to the required erase voltage. Each well sector has at least onememory block formed therein, and thus has a lower capacitance than priorart NAND flash memory arrays that have all memory blocks formed in onelarge well. All the previously described memory bank embodiments andexamples can be used in the NAND memory device of FIG. 1A, or the NANDmemory device chip of FIG. 1B.

As shown by the previous embodiments and examples, a memory array of amemory bank will have reduced well capacitance by forming at least twowell sectors, each well sector having at least one memory block formedtherein. The reduced well capacitance allows for higher erase speeds forthe selected memory block than prior art single well memory arrays. Iferase performance is a non-critical specification for the NAND flashmemory device having multiple well sectors in its memory bank, thecharge pump size can be reduced while maintaining erase speeds similarto that of prior art NAND flash memory devices. In the embodiments andexamples described above, the device elements are connected to eachother as shown in the figures, for the sake of simplicity. In practicalapplications of the present invention to an apparatus, devices,elements, circuits, etc. may be connected directly to each other. Aswell, devices, elements, circuits etc. may be connected indirectly toeach other through other devices, elements, circuits, etc., necessaryfor operation of the apparatus. Thus, in actual configuration, thecircuit elements and devices are directly or indirectly coupled with, orconnected to, each other.

In the preceding description, for purposes of explanation, numerousdetails and examples are set forth in order to provide a thoroughunderstanding of the embodiments of the invention. However, it will beapparent to one skilled in the art that these specific details are notrequired in order to practice the invention. In other instances,well-known electrical structures and circuits are shown in block diagramform in order not to obscure the invention. For example, specificdetails are not provided as to whether the embodiments of the inventiondescribed herein are implemented as a software routine, hardwarecircuit, firmware, or a combination thereof.

The above-described embodiments of the invention are intended to beexamples only. Alterations, modifications and variations can be effectedto the particular embodiments by those of skill in the art withoutdeparting from the scope of the invention, which is defined solely bythe claims appended hereto.

1. A NAND Flash memory comprising: at least two well sectors eachselectively eraseable in response to an erase voltage; at least one NANDcell string in each of the at least two well sectors; and, a bitlineelectrically coupled to the at least one NAND cell string in each of theat least two well sectors.
 2. A method for erasing a memory block in aNAND flash device, comprising: applying an erase voltage to a first wellsector containing a first NAND cell string connected to a bitline forerasing memory cells of the first NAND cell string; and, inhibitingapplication of the erase voltage to a second well sector containing asecond NAND cell string connected to the bitline.
 3. A NAND flash memorydevice, comprising: a memory array having at least two well sectors,each of the two well sectors including NAND cell strings; a charge pumpfor generating an erase voltage; and, a selector for selectively passingthe erase voltage to one of the at least two well sectors.